With the rapid development of the wireless communication technology, the serious shortage of frequency resources has increasingly become a bottleneck which restrains the development of the wireless communication industry. The wireless communication is developing towards the direction of large capacity, high transfer rate and high reliability such that how to maximize the spectrum utilization rate for limited spectrum resources has become a hot subject in current research. With the development of the LTE (Long Term Evolution) industry, currently MIMO antenna systems necessary for 4G have brought new challenges to terminal antenna design and evaluation: on the one hand, users require user experience of miniaturization and high quality, on the other hand, the MIMO antenna systems require that each antenna have the balanced radio frequency and electromagnetic performance while having high isolation and low correlation coefficients. Contradictions in many aspects have been manifested in design and system scheme stages of LTE terminal antennas. To sum up, research achievements in the wireless communication technologies in the past two decades, whether the conventional transmitter diversity or receiver diversity, or the smart antenna technology, is not sufficient to meet today's demand on large channel capacity and high-quality communications. The most important technology used to improve spectrum efficiency or increase communication capacity is the multi-antenna high isolation technology.
The MIMO technology, which is a great breakthrough in the field of wireless mobile communication, is a multi-antenna technology, that is, both a receiver and a transmitter in a wireless communication system are equipped with multiple antennas to create multiple parallel spatial channels, through which multiple information flows are transmitted simultaneously in the same frequency band so as to increase the system capacity greatly and improve the spectrum utilization efficiency. The core idea of the MIMO systems is space-time signal processing, that is, on the basis of the original time dimension, the spatial dimension is increased by using multiple antennas, thereby implementing multidimensional signal processing to obtain spatial multiplexing gain or spatial diversity gain. As an important means to improve the data transfer rate, the MIMO technology attracts people's great concern and is considered as one of alternative key technologies of the future new generation mobile communication systems (4G). Therefore, it has been researched extensively and attracts attention in recent years.
However, up to now, the MIMO technology has seldom implemented commercially in cellular mobile communication systems and is limited by some factors in applications in 3G. One of important factors is the antenna problem. Electrical properties and array configuration of antennas as receiving and transmitting means in the MIMO wireless communication system are important factors that affect the performance of the MIMO system. The number of array elements, array structure, array placement manner, design of antenna units and others directly affect spatial correlation of the MIMO channels. The MIMO system requires that the antenna elements in the array have relatively small correlation so as to ensure that a MIMO channel response matrix is nearly a full rank. However, due to limitations of size and structure of the receiver or transmitter, antenna elements usually are arranged in a limited space as many as possible such that miniaturization of the antennas and coupling between the multiple antennas have become one of problems required to be solved urgently.
Currently, there are many ways to decrease coupling between antennas, such as increasing the antenna spacing; introducing the EBG (Electromagnetic Band Gap) structure; and indenting in the floor. Increasing of the antenna spacing is often limited by the installation volume of antennas in practical applications; both introducing the EBG structure and indenting in the floor require a larger floor, which goes against miniaturization of the antennas as well.